As a radio receiving circuit system for detecting a radio signal and processing the signal, a method using an oscillator is known. With the system, a high frequency oscillator and a quench circuit for controlling the same are included in a signal detector. The high frequency oscillator receives a sawtooth oscillating control signal from the quench circuit, measures an oscillation start time and the like, and detects the presence of an externally-input signal.
Since the quench circuit always operates and inputs a sawtooth oscillating control signal into the high frequency oscillator in the above system, the oscillator oscillates per sawtooth period. Thus, power consumption is large in the oscillator and the above structure is not suitable as a circuit with low power consumption. Further, since a desired signal frequency does not match with an oscillation frequency of the oscillating circuit, the above structure is easily subjected to deterioration in property or effects by interference wave.